The RCA satellite transmits 40 channels, 20 channels of a first polarization and 20 channels polarized 90.degree. relative to the first mentioned 20 channels. Conventionally, two separate Orthomode transitions have been employed in the wave guide for the antenna feed to receive the waves of different polarization, each transition applying the received waves to a different low noise amplifier.
Wave guide Orthomode transitions and low noise amplifiers are large and expensive and as the need for lower cost earth satellite ground stations increases, it is important to reduce both size and cost of components. In copending patent application Ser. No. 168,665 filed on concurrent date herewith for Receive Only Earth Satellite Ground Station assigned to the same assignee as the present invention, there is disclosed an earth satellite ground station in which substantially the entire ground station is disposed about an antenna feed disposed at the focus of a dish antenna. The ability to accomplish this result is dependent at least in part to the ability to employ a single transition and low noise amplifier to receive complentary polarized waves.
Rotation of polarized electromagnetic waves can be accomplished by various techniques such as Faraday rotation using ferrites, but because of the environment in which the present invention is to be employed, such prior art systems are not acceptable due to the high losses introduced by these techniques and instability with temperature and frequency.
U.S. Pat. No. 3,924,205 discloses a horn for a cross-polarized parabolic antenna fed by two wave guides having their longer transverse dimension in a common plane with a 90.degree. pin type polarization rotator located in the feed between the entry points of the two wave guides whereby one wave is rotated 90.degree. relative to the other. Theoretically, the feed of said patent could be used in reverse in the system of the aforesaid application but such would require a long feed and two low noise amplifiers thus defeating the requirement of low cost and small size.